Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (16,743,000)	$ (24,339,000)	$ (20,659,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	3,490,000	2,520,000	3,659,000
Depreciation and amortization	2,104,000	1,861,000	1,570,000
Gain on sale of assets	(860,000)
Loss on disposal of equipment	22,000	262,000	167,000
Write-off of capitalized patent costs	46,000	0	31,000
Increase in inventory reserve	356,000	748,000	700,000
Accretion of interest expense	119,000
Amortization of debt issuance costs	33,000
Amortization of premiums and discounts on investment securities	(33,000)	12,000	10,000
Changes in operating assets and liabilities:
Accounts receivable	(2,798,000)	(1,138,000)	(4,423,000)
Inventories	(4,304,000)	(351,000)	(564,000)
Prepaid expenses and other current assets	(942,000)	1,922,000	(934,000)
Deferred costs	1,268,000	(574,000)	715,000
Other assets	(94,000)	100,000	148,000
Accounts payable	(1,522,000)	2,859,000	(3,495,000)
Accrued expenses and other liabilities	1,122,000	(582,000)	906,000
Deferred revenue	(4,358,000)	1,505,000	(6,030,000)
Net cash used in operating activities	(23,094,000)	(15,195,000)	(28,199,000)
Cash flows from investing activities:
Purchases of equipment, molds, furniture and fixtures	(1,167,000)	(4,880,000)	(5,643,000)
Additions to patent rights	(101,000)	(127,000)	(1,043,000)
Proceeds from sale of assets, net of transaction costs	1,901,000
Proceeds from maturities of investment securities	5,000,000	15,000,000	6,000,000
Purchases of investment securities	(9,964,000)		(15,038,000)
Net cash provided by (used in) investing activities	(4,331,000)	9,993,000	(15,724,000)
Cash flows from financing activities:
Proceeds from issuance of long-term debt	25,000,000
Payment of debt issuance costs	(294,000)
Proceeds from issuance of common stock, net			42,851,000
Proceeds from exercise of stock options	1,816,000	101,000	31,000
Taxes paid related to net share settlement of equity awards	(249,000)	(84,000)	(88,000)
Net cash provided by financing activities	26,273,000	17,000	42,794,000
Effect of exchange rate changes on cash	(1,000)	1,000	(1,000)
Net decrease in cash and cash equivalents	(1,153,000)	(5,184,000)	(1,130,000)
Cash and cash equivalents:
Beginning of year	27,715,000	32,899,000	34,029,000
End of year	26,562,000	27,715,000	32,899,000
Supplemental disclosure of non-cash investing activities:
Purchases of equipment, molds, furniture and fixtures recorded in accounts payable and accrued expenses	40,000	424,000	641,000
Additions to patent rights recorded in accounts payable and accrued expenses	$ 10,000	$ 45,000	$ 21,000